Manipulation of the Asymmetric Swelling Fronts of Photoresist Polyelectrolyte Gradient Thin Films

The depth profile of swelling polyelectrolyte layers is characterized by a static bulk layer and an asymmetric profile with position and shape parameters that describe the intermediate and solution side of the interfacial region. The characteristic width in the solution-side region exceeds the dimensions of the individual chains and therefore is comprised of weakly associated polymers. Contrary to that observed for polyelectrolyte gels and brushes stabilized by cross-links or by covalent bonds to the substrate, respectively, these swelling layers exhibit a more complex response to monovalent and divalent salts. Salt causes an initial contraction of the solution-side interface; layer expansion and polymer dissolution follow at higher salt concentration. The swelling layers measured by neutron reflectivity with mass change verified by quartz crystal microbalance exhibit nonequilibrium responses to the salt concentration, as observed through this interplay between swelling and dissolution. Further, the asymmetric profiles approach, but do not reach, symmetric shapes as expected by mean field equilibrium interfaces. These measurements, motivated by technological needs of photoresist materials, highlight the significance of hydrophobic interactions in determining the structure of associating polymer molecules at the lithographic feature edge.